The present invention relates to the scrambling of video signals, and the like, and more particularly to an image scrambling method and apparatus which is compatible with all video recording, transmitting and processing systems, including signal compression processes.
There is a need for a secure video scrambling method having the property that the scrambled video be compatible with the various video compression systems currently in use. In particular, it should be compatible with compression systems based on for example the Discrete Cosine Transform (DCT), which may employ inter-field redundance coding. Motion picture experts group (MPEG-1 and MPEG-2) are examples of such compression systems. In anticipated applications, the video signal generally is subjected to the processes of scrambling, compression, decompression and descramblingxe2x80x94in that order. The scrambled video therefore will be subjected to the processes of compression and decompression prior to descrambling.
Scrambling systems in use tend to interfere with the natural redundancy in a television signal that the compression systems of previous mention depend upon to realize maximum compression. When redundancy is disturbed, the bit rate requirements for the compression system rise, possibly to an unacceptable level, for the transmission path of the compressed video signal. Therefore there is need for a scrambling system that does not reduce the natural redundancy to a level that requires excessive bit rates to provide a given picture quality.
Prior art scrambling systems using line cut and rotate or line shuffling techniques, for example, are fundamentally incompatible with DCT-based compression methods because they grossly de-correlate information within the DCT pixel matrix. As discussed above, optimal operation of DCT systems is predicated on the existence of a high degree of correlation within each such matrix to effect efficient intra-field compression.
Moreover, the pseudo-random field-by-field changes of the scrambling pattern which these scrambling systems often utilize to increase security, make it impossible to effect further compression by relying on the normal high level of inter-field redundancy of a video signal. For example, scrambling systems based on the process known by the trademark PhaseKrypt, as described in the U.S. Pat. No. 5,058,157, Reissue. 35,078, U.S. Pat. No. 5,438,620, 5,504,815, 5,608,789, 5,581,507 and 5,579,390, all assigned to the same assignee as this application and incorporated herein by reference, in principle also are unsuitable for achieving the purposes of the present invention because they also eliminate this natural inter-field redundancy.
There are various other scrambling methods such as sync suppression and video inversion, for example, which are quite compatible with DCT-based compression. However these methods cannot provide sufficient signal security and therefore also are unsuitable for achieving the purposes of the present invention.
The present invention provides a scrambling method and apparatus which overcomes the incompatibility problems of the prior art scrambling systems of previous mention, while further providing additional compatibility and meeting various desirable requirements.
More particularly, the invention provides a compatibility that lends itself to the prevention of undesirable interactions between the scrambling system and the compression system such as, more specifically;
allowing the compression system to operate on scrambled versus unscrambled video without any increase in the bit-rate needed to provide a given picture quality, and
assuring that system performance (noise, distortion, etc.) is unaffected by the inclusion of the scrambling function.
In addition, the present invention meets requirements such as the following:
The scrambling method and apparatus of the invention is applicable to either analog (NTSC, PAL, SECAM) or digital (CCIR-601) video signals.
The present scrambling method intrinsically provides a high level of resistance to hacking, so that it provides the basis of a secure scrambling system. In other words, it is very difficult to descramble the video by analysis of the scrambled video only.
Also, the present scrambling method admits of a very large number of permutations (scrambling patterns) herein termed xe2x80x9cwarp patternsxe2x80x9d, which ensures the method against trial and error attacks by unauthorized persons. Authorized descrambling techniques and devices are provided with the correct permutation via a designated secure data encryption method.
The present scrambling method primarily is intended for entertainment applications such as video movies, television (TV) shows, and the like. Consequently, it is not essential that the scrambling method effect total concealment of the image. However, sufficient image concealment or distortion is provided so that the entertainment value of the video effectively is eliminated.
In addition to the foregoing requirements and attendant advantages, for maximum operational flexibility, the invention ensures that the scrambled video signal appears as a normal video signal to most if not all video processing methods and devices designed to operate with unscrambled video signals, and the like.
In particular, the video signal scrambled in accordance with the invention is compatible with all video recording, transmitting and processing devices likely to exist in a production or editing environment. Passage of the scrambled video through these devices does not cause any signal degradation (after descrambling) in excess of that caused by passage of unscrambled video through these same devices.
Further, the present scrambling method, or at least a particular embodiment of it. is compatible with consumer video cassette recorders (VCRs). That is, the invention makes it possible to record the scrambled video signal on a consumer grade VCR, and replay it later for descrambling without incurring significant extra signal degradation.
More particularly, the invention provides a library of warp patterns, each of which is capable of imparting a selected degree of local spatial distortion to a video image. A pattern select code determines which stored warp pattern is supplied to a warp engine. The warp engine in turn applies the selected warp pattern to the video image to correspondingly locally spatially distort the image. In a typical application, the thusly scrambled video image and the pattern select code (suitably encrypted) which identifies the pattern, is transmitted via suitable networks such as cable, satellite, etc., to, for example, subscribing users.
The invention includes compatible descrambling of the scrambled signal to authorized users. To this end, the pattern select code is detected in the transmitted scrambled video image and is used to select a descrambling warp pattern, from a plurality of stored warp patterns, which is a complement to the warp pattern used to scramble the original video image. A warp engine applies the complementing warp pattern to the scrambled image to restore it to the original unscrambled video image, thereby allowing the authorized users access to the image.